Its really cool you spent the time to make this, and open source it.
FYI I also have a slicer. It is just a website see https://knicknic.github.io/croppy/ . It works on mobile, windows, mac.
Its a PWA(it even works offline), it uses webassembly compilation of Imagemagick. It supports photoshop files in addition to the formats from whittle. I even have a bookmark you can add so that when you go to webtoons/tapas it auto converts the files as they are uploaded!
I havent updated to support PNG, due to the increased file size what I really want to do is
1 convert the files to png
2 if they exceed 19MB fall back to Jpeg
3 if that exceeds 19MB, force imagemagick to lower quality to a size that will not go too big.
Anyways I will take commits if you want to add anything
Also your app uses bilinear resizing by default. croppy uses Lanczos which is normally better at shrinking images. Also it does not resize for tapas if you upload a picture narrower than 940 for tapas(I do this because some artists just draw at 800 wide)
forgot to add my github page for croppy https://github.com/KnicKnic/croppy and the seperate repo for the extension https://github.com/KnicKnic/croppy-webtoon